This application claims priority from Korean Patent Application No. 2003-23978, filed on Apr. 16, 2003, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a backlight system, and more particularly to an edge-light type backlight system using a light guide panel and a point light source.
2. Description of the Related Art
Backlight systems are used in flat panel displays (FPDs), such as liquid crystal displays (LCDs) to ensure proper luminance or brightness. Backlight systems are classified according to arrangement of light sources as direct-light type backlight units in which a plurality of lamps installed under FPD directly emit light to a liquid crystal panel, and edge-light type backlight units in which a lamp installed along one edge side of a light guide panel emits light to a liquid crystal panel.
Edge-light type backlight systems can use both a rod-shaped light source and a point light source. As a representative rod-shaped light source, there is a cold cathode fluorescent lamp (CCFL) which consists of a tube with electrodes at both ends thereof. As a representative point light source, there is a light emitting diode (LED). CCFLs have many advantages, such as being able to emit strong white light, obtain high luminance and uniformity and being applied to large FPDs. However, CCFLs have disadvantages as well, such as being operated by a high-frequency AC signal and has a narrow operating temperature range. LEDs are inferior to CCFLs when it comes to luminance and uniformity, but have advantages in that they are operated by a DC signal, have a longer life span and a wider operating temperature range, and can be made thin.
FIG. 1 is a schematic perspective view of a related art edge-light type backlight system using a point light source, and FIG. 2 is a side view of the related art edge-light type backlight system of FIG. 1.
Referring to FIG. 1, three LEDs 20 acting as point light sources are installed along one edge surface 11 of a light guide panel 10. A holographic pattern 30 is formed at a bottom surface 13 of the light guide panel 10 to function as an optical path-changing unit which emits light introduced from the LEDs 20 to a light emitting surface 12.
The LEDs 20 emit light toward the edge surface 11 of the light guide panel 10. Since the LEDs 20 are point light sources, they emit light at all angles within ±90 degrees of an optical axis 21. FIG. 3 is a graph of the intensity of light emitted by an LED versus angle with respect to the LED's optical axis. As shown in FIG. 3, the angles at which the intensity is half maximum are about 45 degrees.
Light emitted from the LEDs 20 enters the light guide panel 10 through the edge surface 11. The light is refracted while passing through the edge surface 11, narrowing its spread. When the light guide panel 10 is made of polymethyl-methacrylate (PMMA) and has a refractive index of about 1.49, light propagating through the light guide panel 10 has a spread of ±42 degrees or less. In this case, no light reaches regions indicated by reference numeral 40 between the LEDs 20 directly; light can only reach the regions 40 after several reflections inside the light guide panel 10. Since the amount of reflected light reaching the regions 40 is smaller than the amount of light directly reaching other regions, the regions 40 include dark areas.
When there are dark areas, a screen of an FPD illuminated by the backlight system suffers unevenness in brightness across the areas.
Japanese Patent Publication No. 2001-043717 discloses another edge-light type backlight system using a point light source.